When pressure-filling a bottle, one typically presses its opening up against a filling element to form a seal. Then, a valve opens to let filling material into the bottle. When enough filling material has entered, the valve closes.
In the process of filling, excess liquid filling medium and gas returns to a main supply tank so that it can be reused to fill other bottles. When this material arrives at the tank, it tends to generates a vortex.
A vortex is associated with a region of locally low pressure. Therefore, when a vortex is near a filling medium that has volatile components, those volatile components will be encouraged to come out of solution. This is a particular problem with products that have a high alcohol content, for example alcohol content greater than 25%. Examples of such filing materials include spirits. In such cases, the vortex promotes alcohol evaporation.